Usually, a multitone method (DMT—discrete multitone) is used for asymmetric datastream transmission via normal telephone lines, normal telephone lines usually being constructed as asymmetric digital subscriber lines (ADSL).
An essential advantage of ADSL transmission techniques consists in being able to use conventional cable networks for a transmission, twisted copper pairs normally being used.
High-speed digital subscriber lines of the prior art are described, for example, in the publication “High-speed digital subscriber lines, IEEE Journal Sel. Ar. In Comm., Vol. 9, No. 6, August 1991”.
Among the transmission methods with a high data rate, which are based on digital subscriber lines (DSL), a number of VDSL (Very High Data Rate DSL) arrangements are known and, for example, methods such as carrierless amplitude/phase (CAP), discrete wavelet multitone (DWMT), single line code (SLC) and discrete multitone (DMT) can be used for these. In the DMT method, the transmit signal is provided from multiple sinusoidal or cosinusoidal signals, where both the amplitude and the phase can be modulated of each individual sinusoidal or cosinusoidal signal. The multiple modulated signals thus obtained are provided as quadrature-amplitude modulated (QAM) signals.
FIG. 4 shows a conventional datastream transmitter in which data 123 to be transmitted are input via a data input device 201. The data 123 to be transmitted are supplied to a coding device 202 in which the data are first coded and then assembled to form coded data blocks 125, a predeterminable number of bits to be transmitted being allocated to a complex number depending on the scaling. Finally, the coded data blocks 125 output by the coding device 202 are supplied to a retransformation device 203.
Conventionally, the retransformation device 203 transforms the data present in the frequency domain into the time domain by means of an inverse fast Fourier transform (IFFT), N samples of a transmitter signal being generated directly from N/2 complex numbers, where all N samples will be designated as a discrete multitone (DMT) symbol in the text which follows. The complex numbers can be provided as amplitude values of cosinusoidal and sinusoidal oscillations (real component and imaginary component) to be sent out within a data block, the frequencies being distributed equidistantly in accordance with the relation:
            f      i        =                            i          ·                      1            T                          ⁢                                  ⁢        i            =      1        ,  2  ,      …    ⁢                  ⁢          N      /      2      where T is a period for a transmission of a discrete multitone symbol and N is a number of samples for a discrete multitone symbol. For example, conventional ADSL DMT methods use 256 tones, which can be modulated in amount and phase in each case as sinusoidal tones, in a downstream mode, i.e. in a data transmission from at least one switching center to at least one subscriber. The fundamental frequency is 4.3 kHz and the frequency spacing between successive tones is also 4.3 kHz. Thus, a frequency spectrum from 4.3 kHz (fundamental frequency) to (4.3 kHz+256×4.3 kHz)=1.1 MHz is transmitted. Each DMT symbol is thus represented by a sinusoidal tone which can be modulated in amount and phase, a maximum of 15 bits per symbol usually being represented as complex number. During the transmission of a multitone signal of this type, the problem occurs, however, that transient effects are produced by the transmission channel which, for example, can be constructed as a twisted copper pair, which effects have decayed after, for example, M samples.
In the transmitter device, the last M samples of a DMT symbol are appended to a block start after an inverse fast Fourier transform (IFFT), where the following relation applies: M<N. Due to this cyclic extension (cyclic prefix), a periodic signal can be simulated for the datastream receiver when the transient effect caused by the transmission channel has decayed after M samples and mutual interference between different DMT symbols, i.e. inter-symbol interference (ISI), can be avoided.
As a result, the equalization effort in an equalization device arranged in the datastream receiver can be considerably reduced in conventional methods since after demodulation of the received analog datastream 101 in the datastream receiver, only a simple correction with the inverse frequency response of the transmission channel must be performed in the correction device 112.
A significant disadvantage of a data transmission according to the ADSL method over copper lines in which multitone signals are transmitted consists in that long transient effects occur. The cyclic prefix is thus normally extended in order to supply a periodic signal to the datastream receiver. However, the cyclic prefix must be kept small in relation to the DMT symbol length N, i.e. the following relation must apply:M<<N,since otherwise a reduction in transmission capacity disadvantageously occurs.
In the ADSL standard, a DMT symbol length of N=64 and a value of a cyclic prefix of M=4, for example, is provided for a data transmission from a subscriber to a switch. To limit a transient effect to the cyclic prefix, a special time domain equalizer (TDEQ) in the form of an adaptive transversal filter which operates at a sampling rate Fs (for example 276 kHz in the switching center with ADSL) is provided in the preprocessing device arranged in the datastream receiver in the known method.
Due to the necessary restriction in the length of the cyclic prefix to, for example, M=4, as mentioned above, the quality of transmission is disadvantageously impaired in conventional methods for transmitting an analog datastream 101 since there is still considerable inter-symbol interference (ISI) even when an equalizer is used in the datastream receiver.
A normal transmission channel also disadvantageously contains high-pass and low-pass filters in order to limit the bandwidth of the analog datastream to be transmitted and in order to suppress out-of-band noise in analog-digital and digital-analog converters which can be constructed, for example, as sigma-delta converters.
In particular, it is disadvantageous that when low-pass filters are excited with DMT signals, transient effects occur which have considerable spectral components in a frequency range above the transmission signal band provided. With a sampling rate Fs of, for example, 276 kHz, convolutional products produce spectral components in the transmission signal band which cannot be eliminated by the equalizer arranged in the datastream receiver. These convolutional products are disadvantageously contained as interference signals in the transmission signal band which impairs the quality of transmission.
A multitone signal generated in the time domain is then transmitted in the form of DMT symbols according to FIG. 4. To provide an analog transmitter signal 211, an analog-digital converter is provided for conversion from a digital multitone signal 303 into the analog transmitter signal 211.
A further known datastream transmitter is shown in FIG. 5 and, in addition to the components illustrated in FIG. 4, a first filtering device 131′ and a second filtering device 132′ are here arranged between the retransformation device 203 and the digital-analog converter 204.
FIG. 5 illustrates that a typical transmission channel contains high-pass and low-pass filters for limiting the bandwidth of channel transmission signals. As shown in FIG. 5, the discrete multitone (DMT) symbol 208 is high-pass filtered in the first filtering device 131′ in order to obtain a filtered discrete multitone symbol 209′. This filtered discrete multitone symbol 209′ is low-pass filtered in the second filtering device 132′. The filtering devices 131′ and 132′ used for band limiting have the disadvantage that in the case of excitation with DMT symbols, transient effects occur which limit a data transmission rate. It is particularly if low-pass filters are used as filtering devices 131′ and 132′, respectively, that significant spectral components which, in particular, have an effect with a short cyclic prefix, occur in the frequency range of the signal band. After digital-analog conversion, the time domain signal output by the second filtering device 132′ is finally transferred in a digital-analog converter where transient effects unsuitably occur.